T H E J O U R N A L O F H U M A N R E S O U R C E S • 45 • 1
Small Family, Smart Family?
Family Size and the IQ Scores of Young Men
Sandra E. Black Paul J. Devereux
Kjell G. Salvanes
A B S T R A C T
This paper uses Norwegian data to estimate the effect of family size on IQ scores of men. Instrumental variables IV estimates using sex composition
as an instrument show no significant negative effect of family size; how- ever, IV estimates using twins imply that family size has a negative effect
on IQ scores. Our results suggest that the effect of family size depends on the type of family-size intervention and that there are no important nega-
tive effects of
expected increases in family size. However, unexpected shocks to family size resulting from twin births have negative effects on
the IQ scores of existing children.
I. Introduction
Researchers have long reflected on the role of families in influencing the intelligence of their children. Given evidence that intelligence is in part deter-
mined by the genetic structure of parents, is there room for post-birth family char- acteristics to play a role? If yes, how big is this effect? In this paper, we focus on
the role of one family characteristic: the effect of family size on IQ test scores.
Sandra E. Black is a professor of economics at the University of California, Los Angeles, Paul J. Dev- ereux is a professor of economics at the School of Economics and Geary Institute, University College
Dublin, and Kjell G. Salvanes is a professor of economics at the Norwegian School of Economics. Black and Devereux gratefully acknowledge financial support from the National Science Foundation and the
California Center for Population Research. Salvanes thanks the Research Council of Norway for finan- cial support. The authors would like to thank Josh Angrist and seminar participants at the UCD Geary
Institute, Pompeu Fabra, CEMFS Madrid, the Society of Labor Economists, and the CEPR conference on The Formation and Use of Human Capital and Knowledge in Bergen, Norway for useful suggestions.
The authors are grateful to the Medical Birth Registry for Norway for providing the birth registry data. They are also indebted to Stig Jakobsen who was instrumental in obtaining data access to the IQ data
from the Norwegian Armed Forces. [Submitted March 2004; accepted October 2008]
ISSN 022-166X E-ISSN 1548-8004 䉷 2010 by the Board of Regents of the University of Wisconsin System
34 The Journal of Human Resources
Family size has long been of interest to researchers, particularly because of the strong empirical regularity that children from larger families tend to have poorer
outcomes. There is an extensive theoretical literature on the tradeoff between child quantity and quality within a family that dates back to Becker 1960 and Becker
and Lewis 1973. The theory is often cited and is used as the basis for many macro growth models see Becker and Barro 1988; Doepke 2003. A key element of the
model is an interaction between quantity and quality in the budget constraint that leads to rising marginal costs of quality with respect to family size; this generates
a tradeoff between quality and quantity.
1
But is this tradeoff real? In particular, is it true that having a larger family has a causal effect on the “quality,” or IQ, of the
children? Or is it the case that families who choose to have more children are inherently different, and the children would have lower IQs regardless of family
size?
This paper uses a data set on the male population of Norway to examine the effect of family size on children’s IQ, an outcome not previously available in data sets of
this size. Importantly, we also address the issue of the endogeneity of family size. Until recent years, the empirical literature on the effects of family size on child
outcomes generally relied on ordinary least squares OLS estimation and found a negative relationship between family size and child “quality” usually education,
even after controlling for socioeconomic factors.
2
However, few of these findings can be interpreted as causal; family size is endogenously chosen by parents and
hence may be related to other, unobservable parental characteristics that affect child outcomes.
3
Also, the absence of information on birth order often means that birth order effects are confounded with family-size effects. We are unaware of any studies
of IQ that have attempted to deal with both of these issues. There is, however, a literature on the causal effects of family size on child edu-
cational attainment, starting with Rosenzweig and Wolpin 1980. Recent papers in this literature use arguably exogenous variation in family size induced by the sex
composition of children parents have a preference for variety and so are more likely to have an additional child if the first two are the same sex andor the birth of twins
resulting in a family-size increase of two when one was expected and have gen- erated very little evidence for any quantity-quality tradeoff Conley and Glauber
2006; Caceres-Delpiano 2006; Angrist et al. 2006. In recent work Black, Devereux, and Salvanes 2005, we use Norwegian data on cohorts born between 1912 and
1975 and variation in family size induced by the birth of twins to examine the effects of family size on education and earnings. We find little evidence for any family-
size effect either in the OLS or IV estimates. In this paper, we can study a more recent set of cohorts—all individuals in our sample were born between 1967 and
1987—and are able to introduce previously unavailable information on IQ and birth endowments including birth weight.
1. Rosenzweig and Wolpin 1980 explicitly derive the assumptions under which an exogenous increase in family size should have a negative effect on child quality.
2. See Blake 1989 and the numerous studies cited therein. 3. There is also a literature examining the effect of family size on parental outcomes. See, for example,
Bronars and Grogger 1994 and Angrist and Evans 1998.
Black, Devereux, and Salvanes 35
Given the two types of interventions the instruments represent—one sex com- position is a planned increase in family size based on parental preferences for
variety in the sex composition of their children and the other twin births is an unplanned shock to family size resulting in two generally lower birth weight children
with zero spacing—it is somewhat surprising that researchers so far have found similar effects of family size on child outcomes using the twins and sex composition
instruments for example, Angrist et al. 2006. In contrast, our estimates differ de- pending on estimation method. OLS estimates using a rich set of controls suggest
that there is no strong relationship between family size and IQ. Likewise, IV esti- mates using sex composition as an instrument show no significant negative effect
of family size and are precise enough to rule out large negative effects. However, our IV estimates using twins imply that family size has a negative effect on IQ;
unexpected shocks to family size resulting from twin births have significant negative effects on the IQ of existing children.
Interestingly, this finding is not consistent with our own earlier work. The question then becomes why we observe a negative effect of increases in family size generated
by the birth of twins on IQ when we found little or no effect on education and earnings in our earlier paper. When we examine this issue, we find that this differ-
ence is largely due to the different cohorts studied; when education is studied on a more recent cohort, we find family-size effects similar to those of IQ.
Unlike the papers discussed above, we also have information on birth weight for all individuals in our sample. We use this information to address the recent critique
of twin studies by Rosenzweig and Zhang 2006, who suggest that much of this literature is biased away from finding negative effects of larger families on child
outcomes due to endowment-reinforcing parental investments in children. Impor- tantly, we find that taking account of endowments actually reduces the negative
effects of family size that we find using our basic two stage least squares 2SLS specification, a result that is consistent with compensating rather than reinforcing
parental investments.
The paper unfolds as follows. Section II describes the data we use and Section III discusses our empirical strategy. Section IV presents our results and Section V
discusses the internal and external validity of our estimates as well as potential mechanisms. Section VI compares the estimates with previous estimates from Nor-
way and Section VII concludes.
II. Data
